lk/basics/bash.md

STIN, STOUT, STERR

Input is 0, output is 1, error is 2.

Pipe standard output to log.txt while also outputting it.

cat file.txt |& tee -a log.txt

Copy file and if that's successful, delete it where it stands.

scp archive.tar.gz pi@192.168.0.31:/home/pi && rm archive.tar.gz

A double pipe will try one, and do the other if that fails.

cp -r ~/Archive ~/Backup || tar czf Archive.tar.gz *

REGEX

Regular expression characters include:

\ ^ $ . | ? * + () [] {}

As a result, grep cannot read these characters as literal characters unless they are escaped. E.g.

grep wtf\? log.txt

... will search the string 'wtf?' in the file log.txt. Another version is egrep (now used with 'grep -e') which uses more characters as special characters, or fgrep, which treats all characters as literal strings.

Environmental Variables

PWD, USER, PATH

To display all environmental (but not local) variables, use

env

Set a variable with

colour=red

Display your variable with

echo $colour

Export this to the entire system using:

export colour=blue

Search for Programs

Search for commands relevant to cat.

apropos cat

Show files used in the cat program.

whereis cat

Show which file is the actual code which runs when you type cat:

which cat

Working with Text

Convert every tab to ten spaces.

expand -t 10 file.txt

Or the reverse, with 3 spaces converting to a tab.

unexpand -t 3 file.txt

Format a file by cutting text after 60 characters.

fmt -w 60 file.txt

Indent all but the first line of a paragraph.

fmt -t file.txt

Look at the new lines of a file only:

tail -f /var/log/syslog

The sort function arranges lines alphabetically. Use -r to reverse and -n to sort by number.

Sed

Change all examples of hey to hoi in greetings and show that output (does not change file).

sed 's/hey/hoi/g greetings.txt'

Change each example of 'cat' to 'dog' in the file 'animals.md'.

sed 's/cat/dog/g' animals.md

Measurement

Measure how long a script takes for super-autism powers.

time [bash script]

Aliases

Make one word equal a longer command with alias.

E.g.:

alias wot='sudo systemd-analyze blame && free -h '

Now when you type 'go', it will perform that entire operation.

Functions

Make a function which checks if something is a file, and if so, shows it on screen with cat:

function show(){ file "$1" | grep text &>/dev/null && cat "$1" }

That $1 refers to the first argument typed after the command. If you want to run this on a file called list, then use:

show list

...and the list will output, only if it is text.

In total, this functions the same as typing:

file list | grep text &>/dev/null && list

Remove a function with:

unset my_function

Paths

Every shell has various paths from where it can execute binary files. Find out your current one with:

echo $PATH

To add a directory to a path, e.g. /usr/share/bin, you can declare it in addition to the old path with:

PATH=$PATH:/usr/share/bin

And then check it by echoing the path again.

Before this, probably best to check the path exists with:

if [ -e /usr/share/bin ]; then

echo yes

fi

Pipes, Pedantry and Brackets

Things that statements can do which [ ] statements cannot:

• Intuitive and easy 'and' statements.
  • -z $var && -d ~/LK
• Intuitive and easy 'or' statements.
  • -d LK
• Simple expression comparisons
  • $v1 > $v2
• Simple expression comparisons with clumsy strings
  • item-1 > item-2
• Vague comparisons
  • [[ answer =~ ^y(es)? ]]

exec

exec will start a process running as just that process. In a bash script, the line:

unison rat

... will startup unison as a sub-process of bash. But:

exec unison rat

... starts unison as its own process.

Brace expansion

mv picture{,-1}.jpg

This expands to

mv picture.jpg picture-1.jpg

for Statements

for f in *tiff;do convert "f" "{f/.tiff/.png}" done